Two of the most widely used processes for removal of H.sub.2 S from process gas streams are the catalytic processes that use 1) an iron chelate absorption solution, e.g. the LO-CAT.RTM. process, and 2) a metal vanadate absorption solution, e.g. the Stretford process. The oxidation-reduction reactions that permit these processes to be carried out continuously are well known to those skilled in the H.sub.2 S removal art and are well documented in the literature. The ferric iron chelate-H.sub.2 S reactions can be represented as follows: EQU H.sub.2 S(gas)+H.sub.2 O(Liquid).revreaction.H.sub.2 S(aqueous)+H.sub.2 O(Liquid) EQU H.sub.2 S(aqueous).revreaction.H.sup.+ +HS.sup.- EQU HS.sup.- .revreaction.H.sup.+ +S.sup.= EQU S.sup.= +2(Fe.Chelate).sup.+3 .fwdarw.S.degree. (solid)+2(Fe.Chelate).sup.+2
By combining these equations, the resulting equation is: EQU H.sub.2 S(gas)+2(Fe.Chelate).sup.+3 .fwdarw.2H.sup.+ +2(Fe.Chelate).sup.+2 +S.degree.
In order to have an economical, workable process to effect catalytic oxidation of the hydrogen sulfide using an iron chelate absorption solution, it is essential that the hydrogen sulfide gas be brought continuously into intimate contact with the chelated iron solution and that the ferrous iron chelate formed in the above described manner be continously regenerated by oxidizing to ferric iron chelate by intimate contact with dissolved oxygen, preferably in the form of ambient air. The series of reactions that take place when regenerating the required ferric iron chelate can be represented by the following equations: EQU O.sub.2 (gas)+2H.sub.2 O).revreaction.O.sub.2 (aqueous)+2H.sub.2 O EQU O.sub.2 (aqueous)+2H.sub.2 O +4(Fe.Chelate).sup.+2 .fwdarw.4(OH.sup.-)+4(Fe.Chelate).sup.+3
The economics and workability of the Stretford process have depended upon a large volume of the vanadium solution, and that the reduced metal vanadate, after absorption of the H.sub.2 S (as HS.sup.- and S.sup.=) to form the metal vanadate in the +4 valance state is continously regenerated to the +5 valance state by contact with dissolved oxygen for further use of the oxidized metal vanadate in an absorption zone of the process to remove additional H.sub.2 S as elemental sulfur. The Stretford process chemistry is typically summarized according to the following steps: Absorption and dissociation of H.sub.2 S into alkali: EQU 2H.sub.2 S(g)+2Na.sub.2 CO.sub.3 .revreaction.2NaHS+2NaHCO.sub.3 ;
Bisulfide oxidation with metavanadate to form elemental sulfur and reduced vanadium: EQU 2NaHS+4NaVO.sub.3 +H.sub.2 O.revreaction.Na.sub.2 V.sub.4 O.sub.9 +4NaOH+2S; and
Vanadium reoxidation by dissolved molecular oxygen in the presence of ADA: ##STR1##
The larger catalytic polyvalent metal redox H.sub.2 S absorption processes, e.g. the LO-CAT.RTM. process and the Stretford process, separate the sulfur recovered in the absorber and/or oxidizer chambers of the process by pumping a sulfur/polyvalent metal redox solution slurry to a melting device that melts the sulfur for physical separation of molten sulfur from the catalyst solution and water. The sulfur produced has a purity on the order of 99.9% with ash contents ranging from about 100 to about 400 ppm.
The sulfur recovered from the melter is suitable for many commercial uses, but may be objectional for some markets because 1) the ash content is higher than desirable for sulfuric acid plant catalysts; and 2) there may be inclusions of solid salts in the molten sulfur caused by entrainment of salt-containing catalyst solution in the liquid sulfur leaving the bottom of a sulfur separator (melter).
In addition, there is a troublesome recurrent operating problem in the sulfur melters caused by a combination of excessive salt concentration, higher than normal temperature and excessive residence time. The result is the formation of polyvalent metal polysulfides, e.g. FeS.sub.2 or FeS.sub.3 which deposit as intractable solid material on the melter heat transfer surface and in the sulfur separator.
Some prior art attempts to solve these problems have included 1) filtering the sulfur/redox solution slurry and reslurrying the sulfur with salt-free water; and 2) washing the sulfur, in the form of a cake, on a continuous belt filter using fresh water while removing the wash water from the washed sulfur using a vacuum system prior to reslurrying the sulfur, with salt-free water, and melting the slurry. While these attempts to purify the sulfur product have been successful in achieving higher purity sulfur, the washing and reslurrying steps have added substantial cost to the H.sub.2 S removal, sulfur production process due to the substantial cost of catalytic polyvalent metal redox ingredients lost in the filtering and washing steps. Further, the wash water cannot be returned to the catalytic polyvalent metal redox solution used in the H.sub.2 S absorption process (LO-CAT.RTM. or Stretford processes) without upsetting the water balance required, and making it necessary to withdraw and discard solution to keep the system from overflowing.
The loss of catalytic polyvalent metal redox solution has been greatly reduced or eliminated in accordance with the present invention by recovering the wash water and separating the wash water and redox solution recovered from the sulfur purification and separation steps of the process into re-usable polyvalent metal redox solution and pure water for re-use in washing recovered sulfur.
As a further advantage, the evaporation process and apparatus of the present invention can be operated in such a way to bring about crystallization of metal sulfates, e.g. sodium sulfate, or other salts of limited water solubility, which can be discarded with substantially no loss of catalyst solution.